diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_backlight.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_backlight.c
index 452e772ef136..859cbce2cdaf 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_backlight.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_backlight.c
@@ -494,6 +494,7 @@ amdgpu_dm_register_backlight_device(struct amdgpu_dm_connector *aconnector)
 		drm_dbg_driver(drm, "DM: Registered Backlight device: %s\n", bl_name);
 	}
 }
+EXPORT_IF_KUNIT(amdgpu_dm_register_backlight_device);
 
 void amdgpu_dm_update_connector_ext_caps(struct amdgpu_dm_connector *aconnector)
 {
diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/tests/amdgpu_dm_backlight_test.c b/drivers/gpu/drm/amd/display/amdgpu_dm/tests/amdgpu_dm_backlight_test.c
index 10965306e29d..5a2c32d53487 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/tests/amdgpu_dm_backlight_test.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/tests/amdgpu_dm_backlight_test.c
@@ -440,6 +440,24 @@ static void dm_test_backlight_get_brightness_uses_device_index(struct kunit *tes
 	KUNIT_EXPECT_EQ(test, amdgpu_dm_backlight_get_brightness(bd), 2468);
 }
 
+/* Tests for amdgpu_dm_register_backlight_device() */
+
+/**
+ * dm_test_register_backlight_device_negative_index - Test invalid index no-op
+ * @test: The KUnit test context
+ */
+static void dm_test_register_backlight_device_negative_index(struct kunit *test)
+{
+	struct amdgpu_device *adev = dm_kunit_alloc_adev(test);
+	struct amdgpu_dm_connector *aconnector;
+
+	aconnector = dm_kunit_alloc_connector(test, adev, NULL);
+	aconnector->bl_idx = -1;
+
+	amdgpu_dm_register_backlight_device(aconnector);
+	KUNIT_EXPECT_NULL(test, adev->dm.backlight_dev[0]);
+}
+
 /* Tests for amdgpu_dm_backlight_get_device_index() */
 
 /**
@@ -1616,6 +1634,8 @@ static struct kunit_case dm_backlight_test_cases[] = {
 	KUNIT_CASE(dm_test_backlight_get_level_aux_success),
 	KUNIT_CASE(dm_test_backlight_get_level_aux_error),
 	KUNIT_CASE(dm_test_backlight_get_brightness_uses_device_index),
+	/* amdgpu_dm_register_backlight_device */
+	KUNIT_CASE(dm_test_register_backlight_device_negative_index),
 	/* amdgpu_dm_backlight_get_device_index */
 	KUNIT_CASE(dm_test_backlight_device_index_matches_second),
 	KUNIT_CASE(dm_test_backlight_device_index_missing_fallback),